1. Field of the Invention
This invention relates to a TiAl intermetallic compound-based alloy which exhibits excellent heat resistance, oxidation resistance and resonance resistance, has a cast structure composed of fine equiaxed grains, and is hence suitable for use in the manufacture of rotating components such as the turbine wheels of small-sized superchargers used in passenger cars and trucks, and the turbine blades of large-sized superchargers for ships, jet engines and industrial gas turbines.
2. Description of the Related Art
As a result of a growing interest in environmental problems in recent years, it is desired to enhance the performance of superchargers used in means of conveyance such as passenger cars, trucks and ships, as well as the efficiency of jet engines, industrial gas turbines and the like. Among the various elements constituting the aforesaid products, an important one which governs their performance or efficiency is a turbine. In recent years, several requirements have been proposed for such turbines. They include, for example, an improvement in transient response characteristics, a rise in turbine inlet temperature, and an increase in rotational speed.
The only possible answer to these requirements is an improvement of the materials used for rotating components such as turbine wheels, turbine discs and turbine blades. In order to achieve a rise in turbine inlet temperature, and an increase in rotational speed, an improvement in high-temperature strength (including creep strength) is required, provided that the current Ni-base superalloys are used as starting materials.
However, it has become difficult to further improve high-temperature strength from a compositional point of view. In the present situation, therefore, the focus of investigation has recently shifted to special manufacturing processes involving, for example, conversion into a single crystal. Although such measures may be effectively employed for expensive products manufactured in small quantities, such as turbine blades of jet engines, it is difficult from the viewpoint of cost to employ such special manufacturing processes for mass-produced articles having a complicated shape, such as small-sized superchargers for passenger cars. Moreover, in the case of Ni-base superalloys, it is essentially impossible to achieve an improvement in transient response characteristics by modifying the materials, because the Ni-base superalloys have substantially the same specific gravity (about 8-9), irrespective of composition.
On the other hand, alloys consisting essentially of the intermetallic compound TiAl (hereinafter referred to as TiAl-base alloys) are attracting attention as new metallic materials in recent years. Since these alloys are characterized by light weight (i.e., a specific gravity of about 4) and excellent high-temperature strength, they are promising for meeting the aforesaid three requirements. That is, since their light weight gives a small moment of inertia, it is naturally possible to improve transient response characteristics.
As to the stress loaded on a rotating body, it is only needed to consider its specific strength (i.e., its strength divided by its specific gravity). Since the specific gravity of TiAl-base alloys is one-half of that of Ni-base superalloys, it may be simply said that, if the high-temperature strength of TiAl-base alloys is greater than one-half of that of Ni-base superalloys, a rise in turbine inlet temperature and an increase in rotational speed can be achieved.
As described above, TiAl-base alloys are promising for use as turbine components. However, when their practical use as actual products is taken into consideration, they need to be excellent not only in high-temperature strength, but also in general material characteristics such as fatigue strength, fracture toughness, oxidation resistance and room-temperature ductility. In addition, since the products to be made are rotating components having a complicated shape, the following two characteristics are required.
(1) They must have excellent resonance resistance.
Since a constant driving force acts upon rotating components during service, this may induce resonance. If resonance occurs, the resulting vibrations and noises exceed an allowable level and exert an adverse influence on the environment. In extreme cases, the component may suffer a fatigue failure. It would be difficult to prevent such dangerous resonance solely by design means. Moreover, since the pursuit of this object might produce undesirable effects such as an unduly increased size of the structure, it is a common concept to provide the material itself with the ability to damp vibrations. To this end, the material needs to have high damping power, i.e., great internal friction.
(2) They must have a cast structure formed of fine equiaxed grains.
In many cases, the shapes of the aforesaid rotating components have three-dimensional curved surfaces owing to the required aerodynamic characteristics. Moreover, since the aforesaid rotating components are mass-produced articles, they must be able to be produced in large quantities. Consequently, it is difficult to apply forging and machining processes to these rotating components, and they must be fabricated by precision casting.
In the case of cast articles, their structure is formed during casting, and cannot be easily modified by subsequent heat treatment or the like. Thus, it may be said that the structure is determined by the alloy composition.
If this cast structure is a columnar structure in which the central part of the material solidifies lastly, this is undesirable from the viewpoint of high-speed rotation and reliability because the concentration of impurities and the development of defects tend to occur in the central part which is subjected to the greatest load stress during rotation. Moreover, if the crystal grains of the cast structure are enlarged, the concentration of impurities tends to increase at grain boundaries. Furthermore, in such a case, TiAl-base alloys tend to suffer a transgranular fracture due to cleavage, and hence pose similar problems.
That is, it may be said that a structure in which the above-described problems scarcely arise, namely a structure formed of fine equiaxed grains, is desirable for cast articles for use as rotating components. The desired structure must be produced during casting because, unlike forged articles, cast articles cannot be post-treated by a thermo-mechanical treatment for making the structure finer by recrystallization. To this end, it is necessary to optimize the composition.
Since TiAl-base alloys are attracting attention as new metallic materials of the next generation, they are now being actively investigated all over the world. As a result, it has become possible to improve various properties such as room-temperature ductility and high-temperature strength, by adding suitable alloying elements or optimizing heat-treating conditions.
However, previous investigations on TiAl-base alloys have inclined toward improvements in basis characteristics of materials in general, and no consideration has been given to an improvement in material characteristics which are actually required for practical use on the basis of shape, service environment and the like.
As to the products with which the present invention is concerned, the previously described two requirements for cast rotary components having a complicated shape, i.e., (1) excellent resonance resistance and (2) a cast structure formed of fine equiaxed grains, have scarcely been examined thus far. In other words, it has not been intended in the prior art to improve all of the characteristics required for cast rotating components made of a TiAl-base alloy. Consequently, it may be said that, although TiAl-base alloys have been expected to contribute to an improvement in the performance or efficiency of small-sized superchargers for passenger cars and trucks, large-sized superchargers for ships, jets engines, industrial gas turbines, and the like, it has been difficult to use them for practical purposes in the industrial world.
In view of the above-described existing state of the art, an object of the present invention is to provide a TiAl intermetallic compound-based alloy which not only shows an improvement in general material characteristics such as oxidation resistance, room-temperature ductility and high-temperature strength, but also meets the requirements for its practical use as cast rotating components, i.e., excellent resonance resistance and a cast structure formed of fine equiaxed grains.